1. Field of the Invention
The present invention generally relates to a method for manufacturing liquid crystal display, and more particularly to a method for manufacturing pixel electrodes.
2. Description of the Related Art
Liquid crystal displays (LCDs) have been used for displaying means for watches, calculators and recently television sets and monitors. The thin film transistor-liquid crystal display (TFT-LCD) has an excellent response characteristic and is suitable for a device having high number of pixels, and therefore excellent picture quality and large scale screen comparable to the Cathode Ray Tube monitor can be obtained.
The TFT-LCD as described above, comprises a TFT array substrate arranged with TFTs at an inner surface thereof; a color filter substrate opposed to the TFT array substrate and arranged color filters at an inner surface thereof; and a liquid crystal layer sandwiched between the substrates. In the TFT-LCD as above, the aperture ratio, i.e. a transmittance ratio of a substantially incident light to a dimension of pixel electrode, affects display characteristic of the TFT-LCD. Generally, when the aperture ratio becomes large, the display characteristic is also improved. Accordingly, there have been suggested various structures of TFT-LCD to improve the display characteristic. For example, the top ITO structure has been proposed.
FIG. 1 is a plan view showing a TFT array substrate according to conventional top ITO type. As shown in the drawing, a gate line 2 and a storage line 4 are disposed parallel in a row direction and a data line 8 is disposed to cross over the gate line 2 and the storage line 4. A TFT 10 is arranged at an intersection of the gate line 2 and the data line 8. Herein, the TFT 10 includes a gate electrode elongated from the gate line 2 and a semiconductor layer 6 disposed over the gate electrode with intervening a gate insulating layer (not shown), and source and drain electrodes 9a, 9b disposed on the semiconductor layer 6 and separated with a selected distance.
A transparent conduction layer, for example ITO (Indium Tin Oxide) is disposed at a pixel region defined by the gate line 2 and data line 8. At this time, a pixel electrode 13 is disposed to be overlapped with some portions of the gate line 2 and the data line 8 and also to be in contact with the source electrode 9a.
FIG. 2 is a cross-sectional view taken along II--II in FIG. 1 and a manufacturing method will be described with reference to FIG. 2.
A transparent insulating substrate, for example a glass substrate 1 is provided, and a gate line 2 and a storage line 4 are formed on the glass substrate to be spaced with each other. A gate insulating layer 3 is formed over the glass substrate 1 to cover the gate line 2 and the storage line 4. A semiconductor layer 6 is formed on the gate insulating layer 3 over the gate line 2. A metal layer for data line is formed over the gate insulating layer including the semiconductor layer 6. Next, a data line 8 and source and drain electrodes 9a and 9b are formed by patterning the metal line for data line. As a result, a TFT 10 is fabricated.
An organic insulating layer 11 having a low dielectric constant is formed on the above resultant and a contact hole 12 is formed in the organic insulating layer 11 to expose the source electrode 9a by a known method. Herein, the organic insulating layer 11 is coated with a sufficient thickness to prevent the capacitive coupling that is occurred between the data line 8 and a pixel electrode to be formed later.
A pixel electrode 13 is formed by depositing an ITO within the contact hole 12 and on the organic insulating layer 11 and then patterning the ITO. Herein, the pixel electrode 13 is formed to be overlapped with some portions of the gate line 2 and the data line 8 and to be in contact with the source electrode 9a.
Since the pixel electrode 13 is disposed to be overlapped with the gate line 2 and the data line 8, the TFT-LCD comprising TFT array substrate as described above has a relatively superior aperture ratio to the conventional TFT-LCD that the pixel electrode is disposed only within a pixel region.
However, the pixel electrode has quite a bad reliability due to low adhesion intensity between the ITO and the organic insulating layer.
More particularly, due to the low adhesion between the ITO of an inorganic material and the organic insulating layer, the ITO having relatively thinner thickness compared to the organic insulating layer is severely stressed while pattering. Of the stress applied to the ITO during the patterning process, as shown in FIG. 3, the stress applied to the portion of the ITO which is formed on the organic insulating layer 11, especially the peripheral portion A of the ITO pattern is greater than the stress applied to the portion of the ITO which is contacted with the source electrode 9a which is a signal electrode of the TFT 10 and therefore there is occurred the lift-off phenomenon that the peripheral portion of the ITP pattern is lifted off owing to the stress. As a result, the reliability in the pixel electrode is degraded. Furthermore, in order to prevent the peripheral portion of the ITO from lifting off, line width of the peripheral portion of the ITO formed on the portion of the organic insulating layer 11 which is adjacent to the contact hole 12 should be increased. Therefore, it is very difficult to control the line width of the ITO.